Generally, pilotless aircraft checking and monitoring missions may be of various kinds. Notably, they may relate to the checking of the takeoff or landing phases or else the checking of the proper following of the flight plan during the navigation of such an aircraft.
An operator usually has a display system at his disposal, allowing him to check the behaviour of the aircraft. This system allows the operator to make decisions such as a mission cancellation decision or, if need be, a landing authorization decision or else a continuation of the mission.
In the course of the takeoff or landing phases, the operator must be reactive. In case of incidents, the mission must be rapidly interrupted so as to provide for the safety, firstly, of the onboard personnel in proximity to the landing zone and secondly, of the craft itself.
When the landing or takeoff zone is mobile, a drawback stems from the difficulty of making decisions rapidly to authorize the manoeuvres of a remotely controlled aircraft while guaranteeing maximum safety in the vicinity.
Typically, when the landing takes place on a ship, the swell, the wind and the vertical motions of the aircraft and of the ship may comprise risks in the execution of the manoeuvres.
A deck-landing is a manoeuvre involving numerous risks, notably human risks for the personnel on the ship and hardware risks be they to the aircraft or to the ship in the case of a collision arising from a failed deck-landing.
Authorization for a deck-landing may be given only if all the safety criteria are complied with. The criteria are fixed as a function of each aircraft and of each ship. They may be for example the following:                the deck-landing zone is clear;        the attitudes of the aircraft and of the ship are within the limits, notably yaw, roll, pitch, and speed;        the amplitude of the swell does not exceed a certain limit;        the wind on the deck is favourable, notably as regards its strength and its direction.        
These limits are established on the one hand as a function of the physical characteristics of aircraft notably of their power and of their weight and on the other hand as a function of the capacities of the ship, notably of its size and of the height of the deck. Finally these limits are established for various wind directions and various amplitudes of the swell.
All these limits are established for a given aircraft/ship pair.
The sequence of a landing of a rotary-wing aircraft is not linear, unlike the sequence of a landing of a fixed-wing aircraft. Notably, the expression linear sequence is understood to mean the fact that a landing of a rotary-wing aircraft is tied to a duration for which favourable deck-landing conditions are required. In the landing of fixed-wing aircraft, an authorization is given and it remains valid from the moment the authorization is given.
Concerning fixed-wing aircraft, the aircraft initiates its deck-landing. A decision point makes it possible to verify, before this point, that if all the deck-landing conditions are met then the aircraft continues its deck-landing sequence otherwise it performs a clearance procedure, that is to say it cancels the deck-landing.
Indeed, the deck-landing sequence is generally as follows:                the aircraft nears the deck so as to initiate the deck-landing;        it then regains a position above the deck, situated between 10 to 20 meters to the rear of the deck;        once this position has been regained, it is possible for it to land on deck if the required conditions are all met;        if these conditions are not satisfied, the aircraft then commences a standby phase while hovering;        once the required conditions are satisfied, it is authorized to land on deck.        
Furthermore, the decision to land on deck, subsequent to a hovering phase, is not automatic. It is carried out after validation by an operator who is on the ship.
The operator must therefore ascertain in real time the validity or otherwise of each of the required conditions so as to authorize the deck-landing and thus terminate the phase of hovering above the ship.
A problem encountered in this configuration is that the hovering step is considered to be a risky step, the aircraft being situated above the ship and therefore in a zone close to an infrastructure where personnel are grouped together and being moreover in an aerology disturbed by the ship.
Consequently, it is essential to limit the aircraft's standby duration when it is hovering in a situation ready to land on deck. The operator therefore has the responsibility of authorizing deck-landing as soon as an opportunity arises while ensuring a maximum level of safety. If the operator lacks a slot when the conditions are all met, he must wait for a next slot. A problem is that the next slot in which deck-landing will be possible may arise only a few minutes later in order for all the conditions favourable to deck-landing to be met depending on alterations in these conditions.
Currently, the operator alone evaluates a certain number of parameters whereby the deck-landing of an aircraft may or may not be authorized. There is no device which enables the operator to be afforded a decision aid notably a device which would enable the operator's task to be lightened.
In particular, the deck-landing of a rotary-wing aircraft on a mobile vehicle such as a ship is at present a risky operation.
Currently, the operator on the ship is constrained to evaluate compliance or otherwise with each of the criteria favourable or unfavourable to deck-landing at each instant “t”. When all the criteria are met to authorize a deck-landing, the operator can authorize a deck-landing by actuating a control which allows the aircraft to be given a green light. The aircraft can then go ahead with a procedure for landing on the ship's deck.
If the criteria are poorly judged by the operator through lack of time or because of a human evaluation error, a risk may arise during the deck-landing. On the other hand, if the operator decides to wait for slots in which all the conditions for authorizing a deck-landing are met, he may be caught off guard and not have time to acknowledge the situation so as to transmit a deck-landing directive to the aircraft. Thus one risk is to keep the aircraft hovering in conditions which may cause an additional risk.
A drawback in the latter case is that it increases the time for which the aircraft is hovering above the ship and that it increases the risks of accident when the meteorological conditions are for example fluctuating. It is even possible that the aircraft may quit its hovering situation and leave so as to recommence an approach or find another solution.